The present invention relates to a resist pattern forming method for forming a resist pattern in a lithographic step in a semiconductor device fabrication process.
With LSI higher integration, recently more micronization of circuit patterns are required.
The present micronized pattern forming techniques depend mainly upon light exposure techniques. In the light exposure techniques, with improved efficiency of light exposure apparatuses, higher resist resolving powers have been achieved.
Resist developing treatments are conducted so far dominantly by the wet developing method in which wafers are exposed to developers. This method uses a difference in solution speed between an exposed part of a resist and a non-exposed part thereof. In the case of a positive-type resist, its exposed part is dissolved, whereas its non-exposed part is left. In the case of a negative-type resist, its non-exposed part is dissolved, whereas its exposed part is left.
As the prior methods for forming a pattern of such positive-type resist there have been published, a method comprising the step of heat-treating an exposed film in an atmosphere containing water vapor, at 80.degree.-100.degree. C. for a few to 10 minutes (Japanese Patent Laid-Open Publication No. Sho 52-58374/1977); a method comprising the step of heat-treating an exposed photoresist film in an atmosphere containing water vapor, and the step of developing the photoresist (Japanese Patent Laid-Open Publication No. Hei 04-172461/1992); and a method comprising the step of leaving a substrate for a certain period of time in a humidity-controlled atmosphere to make the water-content of a resin film substantially constant, and the step of irradiating energy beams selectively in a required configuration to develop the film (Japanese Patent Laid-Open Publication No. Sho 59-182442/1984).
The usually generally used positive-type resists are made of novolak resin, and naphthoquinonediazido photosensitizers. Their chemical reaction is this. ##STR1##
When UV radiation (hv) is applied to naphthoquinonediazido, which is insoluble in alkaline developers, the naphthoquinonediazido is photochemically changed into indeneketene. Indeneketene has a short lifetime and quickly reacts with water (H.sub.2 O), which is present in the resist film, into carboxylic acid. Because carboxylic acid is soluble in alkaline developers, the exposed part is dissolved, the non-exposed part remains, and a resist pattern thus formed.
Then it is seen that the amount of the carboxylic acid generated in the exposed part is related to the amount of the water in the resist film. That is, it can be said that the larger the amount of water in the resist film, the larger the amount of carboxylic acid is generated. With a larger amount of carboxylic acid, a solution speed for the exposed part to be dissolved into the developer is higher, in other words, the resist has a higher sensitivity.
But the amount of the water in the resist film has not been controlled, and accordingly improvement of the sensitivity of the resist film has not been expected by methods other than control of the composition (the polarity, the molecular weight distribution, the structure of the sensitizer, etc. of the resin) of the resist. This has been a problem.